Electronic devices such as semiconductor integrated circuits (“ICs”) are typically tested under conditions that include, particularly for complex devices, several temperatures (for example, including temperature extremes). In addition, functional testing is performed on relatively expensive test equipment, and time taken to carry out such testing is important for economic reasons. Thus, time taken to establish test temperatures ought to be minimized to minimize testing time and cost entailed in using such test equipment.
In common testing practice, devices under test (“DUTs”) are brought to a test temperature in a thermal soak step in a test sequence. In more recent testing practice, tests are carried out at several temperatures, such temperatures often including a low temperature that is well below room temperature. The use of such multiple test temperatures, and condensation of water at low temperatures, renders a thermal soak step impractical for helping to establish such multiple test temperatures. In addition, increasingly, DUTs are cycled through a set of test temperatures while they are mounted on a test head, which cycling takes up valuable test equipment time.
Recently, in response to the above, considerable effort has been expended to find methods to establish and cycle DUT temperature rapidly during a test sequence while the DUT is mounted on test equipment (referred to herein as rapid thermal conditioning of a DUT).
One commonly used method to provide rapid thermal conditioning of a DUT entails placing a fluid-cooled test head in contact with a DUT, where the temperature of the test head is modulated by resistive heating, for example, utilizing a resistive heating element attached directly to the test head. In accordance with this method, to raise the temperature of the test head, additional current is passed through the resistive heating element. Likewise, to lower the temperature of the test head, current passed through the test head is reduced. This method is inefficient in that the test head is both heated and cooled at the same time. Further, compromises in design that enable resistive heating to change the temperature of the test head also act to reduce the thermal efficiency of the test head. In particular, the temperature of the resistively heated test head is typically controlled by: (a) sensing the temperature of the DUT, and (b) using this temperature to control current supplied to the test head. In further such embodiments, power supplied to the DUT is also sensed, and the power supplied to the DUT is used to: (a) anticipate temperature changes of the DUT, and (b) accommodate such anticipated temperature changes by adjusting current supplied to the resistive heating element attached directly to the test head.
Another method to provide rapid thermal conditioning of a DUT entails first cooling, and subsequently heating, a stream of air that is directed onto the DUT, where heating of the stream of air is done by resistive heating thereof. Because of this, heating is relatively rapid. In addition, the method of first cooling and then heating a flow of air is inefficient, and limits the efficacy of the method. In further addition, the large amount of energy expended in both heating and cooling the air (or other thermal transfer fluid) limits the amount of air that can be processed in a practical system.
Yet another method to provide rapid thermal conditioning of a DUT entails mixing hot and cold thermal transfer fluids to provide a thermal transfer fluid at a predetermined temperature to a heat exchanger thermally connected to the DUT. In accordance with this method, by adjusting the ratio of flows of the thermal transfer fluids, the temperature of the DUT may be controlled within a band of temperatures between a temperature of the hot thermal transfer fluid and a temperature of the cold thermal transfer fluid. However, because a hot thermal transfer fluid must be mixed with a cold thermal transfer fluid to obtain a thermal transfer fluid having an intermediate temperature, this mixing process is inefficient.
In light of the above, there is a need in the art for method and apparatus that solves one or more of the above-identified problems.